1. Field of the Invention
The present invention relates to a system that relies on radio frequency identification tags (RFID) working with an RFID radar, a directional antenna, a digital compass and a GPS system, in order to locate a lost person, such as a soldier missing in a combat zone.
2. Background of the Prior Art
Radio Frequency Identification (RFID) systems are in common use in a variety of setting such as supply chain management. Shoppers often see the little RFID tags on retail items being purchased. The RFID tags commonly measure in the neighborhood of about 1¼ inches by about ¼ inch and are about 1/16 of an inch thick. The RFID tag contains a receiving antenna, a transmitting antenna, and an integrated circuit chip between the antennas for controlling the device. When a reader is passed over the chip, the reader sends a signal which is received by the receiving antenna, thereby activating the chip. The chip then sends a coded message back to the reader via the transmitting antenna. The circuit chip of the RFID tag uses energy received from the receiving antenna to power the circuit chip as well as the transmitting antenna. This application is referred to as “Passive RFID”. The range (the effective distance between reader and the RFID chip) is limited to only a few feet. If more distance is desired between the reader and the chip (such as in a warehouse, for example), a battery is added on board the RFID tag in order to increase the power of the device. In this active RFID system, the battery is inactive until a signal is received from the reader by the receiving antenna such that the power from the incoming signal turns the battery on to power the outgoing signal. This way, the battery, which is typically smaller than a normal hearing aid battery, is only on for a very short period and its shelf life is approximately 10 years or so. The use of a battery, coupled with a high gain antenna on the reader, can extend the effective range between the reader and the RFID tag to about 500 yards. Active RFID tags also tend to have fewer communication errors between the tag and the reader. A third type of RFID tag is known as a semi-passive system that has an onboard battery, but the battery only provides power to the circuit chip and not to the transmitting antenna. Semi-passive tags are often used in areas where RFID tag measurements are of prime importance such as measurement of the temperature of perishable foods.
Radio transmissions travel at a known speed, namely the speed of light. Radio signals can be bounced off of an object and returned to the transmission point. By measuring the time it takes between the time the signal is dispatched and the time the signal is returned, and knowing the speed of light, the distance between the antenna and the object can be calculated. That's how RADAR works. Therefore, by measuring the time elapsed from the transmission of the signal from the reader to the RFID chip and back to the reader the distance separating the reader and the RFID tag can be precisely calculated.
Antennas that are presently used for RFID systems are normally omni-directional. This means that the antennas project and receive signals from all directions simultaneously, much like automobile radio antennas. Another type of antenna is the unidirectional antenna. An example is the TV dish antenna pointed at a particular satellite. It has a parabolic shape with the antenna set at its focal point. Because it must be pointed at the other antenna with which the unidirectional antenna desires to communicate, this first antenna indicates the direction of the other antenna. It's not necessary that the other antenna also have this shape. Another type of directional antenna is known as the Yagi-Uda or more simply the Yagi antenna, which uses an array of a dipole and a series of parasitic elements, which are typically a reflector and one or more directors. The dipole in the Yagi array is driven, and another element, which is 5% longer than the dipole, operates as a reflector. Other shorter parasitic elements are typically added in front of the dipole as directors. This arrangement gives the Yagi antenna directionality that a single dipole lacks. Yagi antennas are directional along the axis perpendicular to the dipole in the plane of the elements, from the reflector through the driven element and out via the director(s).
The digital compass has become inexpensive and commonplace. For example, many modern vehicles have a digital compass located in the windshield rearview mirror. The digital compass operates by using the “Hall Effect” based on the understanding of the angles at which the earth's magnetic field enters the earth. By using three Hall Effect transistors on the same chip, it can make up for differences induced by tilt up, down or sideways. The digital compass is very accurate in determining which way it's pointed.
Twenty-three satellites circle the earth sending timed signals. These satellites are called Global Positioning Satellites (GPS). A GPS receiver on earth that receives signals from at least three of these satellites can determine its location to within a few feet.
By using an RFID system that utilizes RFID Radar, a directional antenna, a digital compass and an appropriate algorithm, we can determine the location of a missing soldier with an RFID chip in his or her gear by determining the direction and distance from the reader instrument to the his chip. By combining this direction and distance information with the GPS coordinates of the reader, the ability to accurately determine the GPS coordinates, map coordinates and/or latitude/longitude of the missing soldier or other lost person can be achieved.